Concentrates of blood platelets in plasma are infused into patients to treat bleeding problems. Human platelets can withstand storage for only up to five days before progressive loss of platelet viability negates further clinical use. The lifetimes of stored platelets are highly variable, so it is inevitable that some useless platelets are given to patients while other packs of viable platelets are disposed of needlessly at the end of their arbitrary shelf life. Unfortunately, there is not in use any test of platelet viability which can be used without removing a sample of platelets from the pack. This is inconvenient, time consuming and involves the risk of the pack contents becoming infected.
Recently, an optical technique has been proposed for the measurement of platelet viability without withdrawing samples from the pack. The technique depends on the light scattering properties of the platelets which differ when the platelets are functional or dead. It is believed that this effect stems from the change of shape of platelets during storage. Functional platelets have a discoid shape. As the platelets age, more and more of them lose their discoid shape and become nearly spherical, thereby changing their light scattering property. The previously proposed technique has involved a comparison of the light scattering properties of the platelets when they are flowing and when they are stationary. Thus, when a beam of light is incident on a pack of the platelets, it has been found that the ratio of the intensity of light scattered in a particular direction when the platelets are moving, divided by the intensity of light scattered in the same direction when the platelets are still, may be less than 0.7 when the platelets are functional but approaches unity as the platelets die. This technique is of theoretical interest but its application would be difficult in clinical use as it would be necessary to remove a pack of platelets from the rocking tray, on which such packs are normally stored to keep the platelets in motion, and to transfer it to some device in which the light scattering properties could be measured both when the platelets are in motion and stationary.